V Verilogu je vedno blok eden izmed postopkovnih blokov. Stavki znotraj vedno bloka se izvajajo zaporedno.
Vedno blok se vedno izvede, za razliko od začetnih blokov, ki se izvedejo samo enkrat na začetku simulacije. Blok vedno mora imeti občutljiv seznam ali z njim povezano zakasnitev
Občutljiv seznam je tisti, ki bloku vedno pove, kdaj naj izvede blok kode.
Sintaksa
pretvorba niza v celo število v Javi
The Verilog vedno blokira naslednjo sintakso
always @ (event) [statement] always @ (event) begin [multiple statements] end
Primeri
Simbol @ za rezervirano besedo nenehno , pomeni, da se bo blokada sprožila pri pogoj v oklepaju za simbolom @.
always @ (x or y or sel) begin m = 0; if (sel == 0) begin m = x; end else begin m = y; end end
V zgornjem primeru opisujemo mux 2:1 z vhodoma x in y. The to je izbrani vnos in m je izhod muxa.
V kateri koli kombinacijski logiki se izhod spremeni vsakič, ko se spremeni vhod. Ko se ta teorija uporabi za vedno bloke, je treba kodo znotraj vedno blokov izvesti vsakič, ko se spremenijo vhodne ali izhodne spremenljivke.
OPOMBA: Poganja lahko vrste podatkov reg in integer, ne more pa poganjati podatkovnih tipov žice.
V Verilogu obstajata dve vrsti občutljivih seznamov, na primer:
java scan.nextstring
- Nivo občutljiv (za kombinacijska vezja).
- Občutljivo na rob (za japonke).
Spodnja koda je isti 2:1 mux, vendar izhod m je zdaj flip-flop izhod.
always @ (posedge clk ) if (reset == 0) begin m <= 0; end else if (sel="=" 0) begin m <="x;" pre> <h4>NOTE: The always block is executed at some particular event. A sensitivity list defines the event.</h4> <h3>Sensitivity List</h3> <p>A sensitivity list is an expression that defines when the always block executed, and it is specified after the @ operator within the parentheses ( ). This list may contain either one or a group of signals whose value change will execute the always block.</p> <p>In the code shown below, all statements inside the always block executed whenever the value of signals x or y change.</p> <pre> // execute always block whenever value of 'x' or 'y' change always @ (x or y) begin [statements] end </pre> <p> <strong>Need of Sensitivity List</strong> </p> <p>The always block repeats continuously throughout a simulation. The sensitivity list brings a certain sense of timing, i.e., whenever any signal in the sensitivity list changes, the always block is triggered.</p> <p>If there are no timing control statements within an always block, the simulation will hang because of a zero-delay infinite loop.</p> <p>For example, always block attempts to invert the value of the signal clk. The statement is executed after every 0-time units. Hence, it executes forever because of the absence of a delay in the statement.</p> <pre> // always block started at time 0 units // But when is it supposed to be repeated // There is no time control, and hence it will stay and // be repeated at 0-time units only and it continues // in a loop and simulation will hang always clk = ~clk; </pre> <p>If the sensitivity list is empty, there should be some other form of time delay. Simulation time is advanced by a delay statement within the always construct.</p> <pre> always #10 clk = ~clk; </pre> <p>Now, the clock inversion is done after every 10-time units. That's why the real Verilog design code always requires a sensitivity list.</p> <h4>NOTE: Explicit delays are not synthesizable into logic gates.</h4> <h3>Uses of always block</h3> <p>An always block can be used to realize combinational or sequential elements. A sequential element like flip flop becomes active when it is provided with a clock and reset.</p> <p>Similarly, a combinational block becomes active when one of its input values change. These hardware blocks are all working concurrently independently of each other. The connection between each is what determines the flow of data.</p> <p>An always block is made as a continuous process that gets triggered and performs some action when a signal within the sensitivity list becomes active.</p> <p>In the following example, all statements within the always block executed at every positive edge of the signal clk</p> <pre> // execute always block at the positive edge of signal 'clk' always @ (posedge clk) begin [statements] end </pre> <h3>Sequential Element Design</h3> <p>The below code defines a module called <strong> <em>tff</em> </strong> that accepts a data input, clock, and active-low reset. Here, the always block is triggered either at the positive edge of the <strong> <em>clk</em> </strong> or the negative edge of <strong> <em>rstn</em> </strong> .</p> <p> <strong>1. The positive edge of the clock</strong> </p> <p>The following events happen at the positive edge of the clock and are repeated for all positive edge of the clock.</p> <p> <strong>Step 1:</strong> First, if statement checks the value of active-low reset <strong> <em>rstn</em> </strong> .</p> <ul> <li>If <strong> <em>rstn</em> </strong> is zero, then output q should be reset to the default value of 0.</li> <li>If <strong> <em>rstn</em> </strong> is one, then it means reset is not applied and should follow default behavior.</li> </ul> <p> <strong>Step 2:</strong> If the previous step is false, then</p> <ul> <li>Check the value of d, and if it is found to be one, then invert the value of q.</li> <li>If d is 0, then maintain value of q.</li> </ul> <pre> module tff (input d, clk, rstn, output reg q); always @ (posedge clk or negedge rstn) begin if (!rstn) q <= 0; else if (d) q <="~q;" end endmodule pre> <p> <strong>2. Negative edge of reset</strong> </p> <p>The following events happen at the negative edge of <strong> <em>rstn</em> </strong> .</p> <p> <strong>Step 1:</strong> First, if statement checks the value of active-low reset <strong> <em>rstn</em> </strong> . At the negative edge of the signal, its value is 0.</p> <ul> <li>If the value of <strong> <em>rstn</em> </strong> is 0, then it means reset is applied, and output should be reset to the default value of 0.</li> <li>And if the value of <strong> <em>rstn</em> </strong> is 1, then it is not considered because the current event is a negative edge of the <strong> <em>rstn</em> </strong> .</li> </ul> <h3>Combinational Element Design</h3> <p>An always block can also be used in the design of combinational blocks.</p> <p>For example, the digital circuit below represents three different logic gates that provide a specific output at signal o.</p> <img src="//techcodeview.com/img/verilog-tutorial/39/verilog-always-block.webp" alt="Verilog Always Block"> <p>The code shown below is a module with four input ports and a single output port called o. The always block is triggered whenever any of the signals in the sensitivity list changes in value.</p> <p>The output signal is declared as type <strong> <em>reg</em> </strong> in the module port list because it is used in a procedural block. All signals used in a procedural block should be declared as type <strong> <em>reg</em> </strong> .</p> <pre> module combo (input a, input b, input c, input d, output reg o); always @ (a or b or c or d) begin o <= ~((a & b) | (c^d)); end endmodule < pre> <p>The signal o becomes 1 whenever the combinational expression on the RHS becomes true. Similarly, o becomes 0 when RHS is false.</p> <hr></=></pre></=></pre></=>
Potreben seznam občutljivosti
Blok vedno se neprekinjeno ponavlja skozi celotno simulacijo. Seznam občutljivosti prinaša določen občutek časovne razporeditve, tj. kadar koli se spremeni kateri koli signal na seznamu občutljivosti, se sproži vedno blok.
sonu nigam
Če znotraj bloka vedno ni stavkov za krmiljenje časa, se bo simulacija ustavila zaradi neskončne zanke brez zakasnitve.
Na primer, vedno blokiraj poskuse obračanja vrednosti signala clk. Stavek se izvede po vsakih 0-časovnih enotah. Zato se izvaja večno, ker v stavku ni zamude.
// always block started at time 0 units // But when is it supposed to be repeated // There is no time control, and hence it will stay and // be repeated at 0-time units only and it continues // in a loop and simulation will hang always clk = ~clk;
Če je seznam občutljivosti prazen, bi morala obstajati kakšna druga oblika časovne zakasnitve. Čas simulacije se podaljšuje s stavkom zakasnitve znotraj konstrukcije always.
always #10 clk = ~clk;
Zdaj se inverzija ure izvede po vsakih 10-časovnih enotah. Zato prava Verilogova oblikovalska koda vedno zahteva seznam občutljivosti.
OPOMBA: eksplicitnih zakasnitev ni mogoče sintetizirati v logična vrata.
Uporaba vedno blokiraj
Vedno blok se lahko uporablja za realizacijo kombinacijskih ali zaporednih elementov. Zaporedni element, kot je flip flop, postane aktiven, ko je opremljen z uro in ponastavitvijo.
Podobno postane kombinacijski blok aktiven, ko se spremeni ena od njegovih vhodnih vrednosti. Vsi ti bloki strojne opreme delujejo sočasno neodvisno drug od drugega. Povezava med vsakim določa pretok podatkov.
poveži bazo podatkov java
Vedno blok je narejen kot neprekinjen proces, ki se sproži in izvede določeno dejanje, ko signal na seznamu občutljivosti postane aktiven.
V naslednjem primeru se vsi stavki znotraj bloka always izvedejo na vsakem pozitivnem robu signala clk
// execute always block at the positive edge of signal 'clk' always @ (posedge clk) begin [statements] end
Oblikovanje zaporednih elementov
Spodnja koda definira modul, imenovan tff ki sprejema vnos podatkov, uro in aktivno nizko ponastavitev. Tukaj se vedno blok sproži na pozitivnem robu clk ali negativni rob rstn .
1. Pozitivni rob ure
Naslednji dogodki se zgodijo na pozitivnem robu ure in se ponavljajo za vse pozitivne robove ure.
Korak 1: Najprej stavek if preveri vrednost aktivne nizke ponastavitve rstn .
- če rstn je nič, potem je treba izhod q ponastaviti na privzeto vrednost 0.
- če rstn je ena, potem to pomeni, da ponastavitev ni uporabljena in mora slediti privzetemu vedenju.
2. korak: Če je prejšnji korak napačen, potem
- Preverite vrednost d in če se ugotovi, da je ena, obrnite vrednost q.
- Če je d 0, ohranite vrednost q.
module tff (input d, clk, rstn, output reg q); always @ (posedge clk or negedge rstn) begin if (!rstn) q <= 0; else if (d) q <="~q;" end endmodule pre> <p> <strong>2. Negative edge of reset</strong> </p> <p>The following events happen at the negative edge of <strong> <em>rstn</em> </strong> .</p> <p> <strong>Step 1:</strong> First, if statement checks the value of active-low reset <strong> <em>rstn</em> </strong> . At the negative edge of the signal, its value is 0.</p> <ul> <li>If the value of <strong> <em>rstn</em> </strong> is 0, then it means reset is applied, and output should be reset to the default value of 0.</li> <li>And if the value of <strong> <em>rstn</em> </strong> is 1, then it is not considered because the current event is a negative edge of the <strong> <em>rstn</em> </strong> .</li> </ul> <h3>Combinational Element Design</h3> <p>An always block can also be used in the design of combinational blocks.</p> <p>For example, the digital circuit below represents three different logic gates that provide a specific output at signal o.</p> <img src="//techcodeview.com/img/verilog-tutorial/39/verilog-always-block.webp" alt="Verilog Always Block"> <p>The code shown below is a module with four input ports and a single output port called o. The always block is triggered whenever any of the signals in the sensitivity list changes in value.</p> <p>The output signal is declared as type <strong> <em>reg</em> </strong> in the module port list because it is used in a procedural block. All signals used in a procedural block should be declared as type <strong> <em>reg</em> </strong> .</p> <pre> module combo (input a, input b, input c, input d, output reg o); always @ (a or b or c or d) begin o <= ~((a & b) | (c^d)); end endmodule < pre> <p>The signal o becomes 1 whenever the combinational expression on the RHS becomes true. Similarly, o becomes 0 when RHS is false.</p> <hr></=></pre></=>